Crescent Moon: Its Bright Side Points to the Sun

Follow the curve, not the tips

The bright side of a crescent Moon faces the Sun. You can use it to estimate the Sun’s direction, even shortly after sunset. The overlooked detail: the two pointed ends are not your pointer. Look at the middle of the bright, rounded outer edge instead.

Try it at dusk

1. Find the Moon when its bright and dark areas are clearly distinguishable. A crescent or half Moon works well.
2. Imagine a line from the centre of the lunar disc through the middle of its bright, curved outer edge.
3. Continue that direction mentally across the sky. It leads roughly towards the Sun; after sunset, that means below the horizon.
4. Compare your estimate with the sunset position or a sky map app.

Why it works

Moonlight is reflected sunlight. The Sun normally illuminates half of the Moon. Moon phases happen because you see different amounts of that illuminated half as the Moon orbits Earth. The bright curve is therefore on the sunward side. The usual dark portion is not Earth’s shadow: that shadow matters during a lunar eclipse.

A clue, not a precision compass

The sky appears to curve around you. A direction indicated by the Moon will not always extend as a straight line all the way to the Sun in a photograph. A very thin crescent or a Moon close to the horizon can make the geometry look especially odd. A full Moon lacks a clear sideways pointer, while a new Moon is barely visible. Use this for a rough estimate, not navigation. Never look directly at the Sun to check, especially through binoculars or a telescope.